Hi Chris_K,
The problem is not only related to the grayed-out option, but also to the fact that the performance with metal is equal or even worse than with the other available options. At first glance, it seems as if Metal doesn't use any hardware acceleration in Affinity Photo with the specified graphics cards, although the graphics cards could.
In other words, the announced metal-supported performance of Affinity Photo on a MacBook Pro is quite impressive, while on the iMac it's simply bad although the graphics cards are with macOS native metal-supported.
Regards,
Gerald

Hi Chris_K,
thanks for the fast answer. I've tested the Metal-Option on my two macOS-Systems.
System 1:
iMac 5k (late 2014) with an AMD Radeon R9 M295X 4GB (GPU supports Metal)
macOS High Sierra 10.13.1
Display: Metal
-> No hardware acceleration option in AP available?!
System 2:
Macbook Pro 15" (late 2016) with an Intel HD 530 and AMD Radeon Pro 460 4GB (GPU supports Metal)
macOS High Sierra 10.13.1
Display: Metal
-> Hardware acceleration option in AP available
As soon as AP has been started on system 2, the system switches to the AMD GPU. Test with the lightning filter showed a filter representation smoothly in real time without artifacts (like with AP on the latest iPad Pro). The CPU load was about 20% while the filter moved wildly.
On system 1, the same filter showed artifacts and jerked noticeably. The CPU load temporarily reached 100% while the filter was moved.
I deactivated the hardware acceleration in System 2 in the settings and promptly showed the same behavior as in System 1 (high CPU load and artifacts while the filter is moved).
If it can be deduced from the tests, AP does not use any metal hardware acceleration in System 1, although the hardware allows this. I guess this is a clear bug in AP
Kind regards,
Gerald